Operational Comparison of Transit Signal Priority Strategies

Transcription

1 Operational Comparison of Transit Signal Priority Strategies Revision Submitted on: November, 0 Author: Adriana Rodriguez, E.I Assistant Engineer Parsons Brinckerhoff 0 South Orange Avenue, Suite 00 Orlando, FL 0 Office Phone: (0) - Fax: (0) -0 Mobile Phone: (0) - rodriguezam@pbworld.com Co-Author: Alan R. Danaher, P.E., PTOE, AICP, PTP Senior Supervising Engineer Parsons Brinckerhoff 0 South Orange Avenue, Suite 00 Orlando, FL 0 Office Phone: (0) - Fax: (0) -0 Mobile Phone: (0) - danaher@pbworld.com Word Count:, TRB 0 Annual Meeting

2 0 ABSTRACT Priority strategies for bus transit are implemented with the purpose of expediting transit flow by reducing travel times and delay while providing a more reliable system that may attract new riders and increase transit capacity. Such strategies are a critical component of Bus Rapid Transit (BRT), though can be also applied to regular local bus operations. The interference of general traffic when buses are operating in mixed traffic results in a reduction in speed and capacity. Intersection priority treatments reduce signal delay resulting in travel time savings to transit. Transit Signal Priority (TSP) strategies may be applied across a number of intersections depending on the level of service (LOS) and lane configuration characteristics of the signalized intersections along a corridor and can also be combined in the same signal operation for each approach serving transit. This paper presents the methodology for evaluating the impacts of TSP treatments on transit operations at a specific intersection by comparing various treatment options. The effectiveness of the transit priority treatment is measured by the travel time savings for bus operations given in minutes of reduced delay. The applicability of this methodology in the planning of TSP strategies for transit service is explained in two sample calculations where the results are compared to determine the feasibility of the TSP treatment that would give the highest travel time savings for signalized intersections along the corridor being studied. 0 0 TRB 0 Annual Meeting

3 Operational Comparison of Transit Signal Priority Strategies Introduction Priority strategies for bus transit are implemented with the purpose of expediting transit flow by reducing travel times and delay while providing a more reliable system that may attract new riders and increase transit capacity. Such strategies are a critical component of Bus Rapid Transit (BRT), though are being more commonly applied to regular local bus operations. Priority measures are successfully implemented in areas with the following characteristics: Intensively developed downtown area with limited street capacity Areas with long-term reliance on public transportation Highway capacity limitations on the approaches to downtown and along other major arterials Alternate Intersection Priority Treatments The interference of general traffic when BRT is operating in mixed traffic results in a reduction in speed and capacity. Intersection priority treatments reduce signal delay resulting in travel time savings to transit. Signal priority strategies may be applied across a number of intersections depending on the level of service (LOS) and lane configuration characteristics of the signalized intersections along a corridor and can also be combined in the same signal operation for each approach serving transit. Mainline TSP Transit Signal Priority (TSP) along the mainline of the roadway is implemented by adjusting the signal timing to give priority to transit vehicles. It consists of a minor modification of the phase split times that extends the green phase serving the approaching bus reducing the bus delay. The extra time for transit is accommodated by a slight reduction in green time for minor movements (typically the minor street or major street left turns), with cycle length maintained to preserve corridor signal coordination. Figure illustrates the implementation of TSP at a signalized intersection. TSP can be manually triggered by the bus driver or more typically can be automatically controlled using on-board Automatic Vehicle Location (AVL) or Automatic Passenger Counter (APC) technology. Table describes the differences in TSP treatments. TSP strategies include: Passive Priority: Accommodate buses by using pre-timed modifications to the signal timing that occur even when the bus is not present. Active Priority: Adjustments to the signal timing are triggered by the bus detection when the bus is approaching the intersection. Active priority may be unconditional which gives priority whenever the bus arrives or conditional which activates priority only if certain conditions are met related to the transit vehicle, such as whether the bus is behind schedule or if there are a certain number of passengers on the bus. Real-Time Priority: Adjusts the signal timing based on general traffic arrivals at the particular intersection or at a network of intersections (so-called actuated signal control). TRB 0 Annual Meeting

4 TABLE TSP Treatments Passive Priority Active Priority Treatment Adjust cycle length Split phases Areawide timing plans Bypass metered signals Phase extension Early start Special phase Phase suppression Description Reduce cycle lengths at isolated intersections Apply multiple phases while maintaining original cycle length Preferential progression for buses through signal offsets Buses operate in exclusive lanes, special signal phases Increase phase time Reduce other phase times Addition of a bus phase Skipped non-priority phases Real Time Priority Delay-optimizing control Network control Signal timing changes to reduce overall person delay Signal timing changes considering the overall system performance Source: Transit Capacity and Quality Service Manual, TCRP Report, 00 FIGURE TSP Concept TRB 0 Annual Meeting

5 0 Queue Jump Signal Queue jump treatments reduce delay in bus transit at signalized intersections by using a right-turn lane or a separate lane for buses only to avoid the queue in the adjoining through lanes. Buses are exempted from any right-turn requirements at the intersection. A bus signal phase provides a green indication for a short period of time (usually to seconds) before the adjacent traffic lanes get their green indication that allows the bus to exit the auxiliary lane and merge into general traffic farside of the intersection. Queue jump treatments work most effectively at a bus stop when that stop is located near side of an intersection to allow the bus to trigger the bus signal phase after it serves the stop. The green time for the general traffic movement is reduced to accommodate the bus signal phase. Figure illustrates the queue jump signal concept for bus service. 0 Source: TCRP Report 00, 00 FIGURE Queue Jump Concept TRB 0 Annual Meeting

6 0 0 0 Issue of Appropriate Application TSP can be applied at a single intersection or at various intersections along the corridor; TSP is most effective at signalized intersections operating under LOS C or D with a volume to capacity (v/c) ratio between 0.0 and.0. TSP is not applicable for intersections operating with a v/c ratio greater than.0 due to the long vehicle queues that prevent buses to clear the intersection soon enough to take advantage of the extended green time. TSP is usually implemented with stops located on the far side to allow activating the priority call and clear the intersection before reaching the stop. Queue jumps are an alternative to TSP and are preferable when there is a need for a near side bus stop or where TSP is not applicable due to LOS constraints. It is also important to consider appropriate length of the auxiliary lane used for the queue jump and any general traffic sharing the lane (typically right turn traffic) to assure the effectiveness of the queue jump treatment. Evaluation Process The process of evaluating the impacts of TSP treatments on transit operations is based on the comparison of various treatment options for the alternatives being evaluated. The effectiveness of the transit priority treatment is measured by the travel time savings for bus operations given by a reduction in delay for specific intersections vs. minutes per mile for multiple treatments along a corridor. The travel time savings is an important factor in determining the local preferred alternative for bus operations. The flow chart shown in Figure illustrates the evaluation process in the planning of TSP strategies. To assure the effectiveness of implementing TSP treatments, the following factors should be evaluated (TCRP Report, 00 and TCRP Synthesis, 00): Is the transit priority strategy feasible under the current and projected traffic conditions and bus volumes? Can the transit priority strategy be implemented without significantly increase in congestion on heavily traveled cross streets? Would the transit priority strategy provide added benefits to warrant the added cost? Can bus stops be located (far side or near side) to allow effective operations depending on the applicability of the transit priority treatment? Is the existing traffic control system capable of handling the proposed transit priority treatment? Screening Level traffic operations / Signalized intersection analysis Identify intersections where TSP and/or Queue Jump is Operationally Feasible Identify transit travel savings under base conditions Select preferred priority treatment Evaluate impact of improvement strategy FIGURE Transit Signal Priority Evaluation Process Identify lane configuration modifications to improve savings TRB 0 Annual Meeting

7 0 0 Initial Intersection Screening The process starts by identifying key intersections along the corridor being evaluated and determining the level of service for existing and future conditions. Once the level of service (LOS) has been identified, the locations where signal priority can be implemented are determined. Locations with LOS C or D are selected for further evaluation; signalized intersections operating within this range of level of service have shown to provide the optimal conditions for achieving significant travel time savings to transit operations with TSP without disrupting general traffic operations. The existing configuration of the intersections is examined to determine locations with exclusive right turn lanes or where there is an opportunity to develop a separate bus-only lane for further evaluation; such a lane provides the opportunity for queue jump implementation without major modifications to the existing lane configuration to achieve significant travel time savings for bus operations. TSP Delay Reduction Estimation The signalized intersections operating at a LOS C and D are selected for TSP implementation analysis. The impacts of TSP implementation are analyzed to determine the approach delay variation when the green time is extended for the approach serving bus traffic. The v/c ratio is also monitored for variations to make sure that the time assigned for the bus signal does not result in a v/c ratio approaching or greater than which indicates a negative effect in traffic operations. The difference in delay for the approach where a bus is operating translates in seconds of travel time savings. The green time extension is then recovered through reductions in green time for other minor intersection movements to avoid disruption in the traffic signal coordination along the corridor. The seconds assigned to the green time extension are assumed to be equal to the estimated time needed for buses to clear the intersection operating under the identified conditions. For overall corridor analysis, the travel time savings can be given on a per mile basis and the analysis performed segment by segment in between intersections or in between bus stops. Figure illustrates the estimation of delay or travel time savings in seconds per vehicle with and without TSP implementation at a signalized intersection under a particular g/c (green time/cycle length) conditions. The delay curves relate the g/c ratio to the v/c of the approach allowing the comparison of delays for the initial g/c and estimate the savings with TSP implementation. TRB 0 Annual Meeting

8 0 Source: TCRP Report, 00 FIGURE Effect of TSP on Signalized Intersection Delay (0-Second Cycle) Queue Jump Delay Reduction Estimation The signalized intersections with applicable auxiliary lanes are selected for queue jump traffic operation analysis. Buses are assumed to use the right turn lane or a separate lane to avoid the queue in the through traffic allowing the bus to clear the intersection and merge before the green signal for general traffic. The adjacent through lane th percentile queue length is also examined to determine if the queue length in the adjacent through lane would extend to pass the right turn or separate lane storage length restricting buses from use of the auxiliary lane. The difference in delay for the general traffic lane and the auxiliary lane translates in seconds of travel time savings; for purpose of the analysis, the travel time savings is given in minutes per mile and the analysis is performed segment by segment in between intersections or in between bus stops for modeling purposes. Figure shows the travel time savings related to queue jump implementation at a signalized intersection, assuming that the storage lane has the appropriate length for effective operations and the assigned green time for the advance green is approximately 0% of the cycle length. TRB 0 Annual Meeting

9 0 0 Source: TCRP Report, 00 FIGURE Effect of Queue Jump with Advanced Green on Signalized Intersection Delay (0-Second Cycle) Comparison Analysis The transit priority strategies implementation analysis may result in signalized intersection locations where transit signal priority and queue jumps are both applicable based on the level of service at which the interaction is operating and the existence of an exclusive right turn or separate lane. The seconds of travel time savings per approach are compared and the option given the highest savings is identified from an operational perspective as the preferred transit priority strategy to be implemented at a particular approach. The analysis can also be performed for future conditions applying a growth factor to the traffic volumes for the intersections being analyzed; the growth factor is determined based on the projections for the increase in traffic per approach for a future year. The future year conditions analysis helps identify locations where the increase in traffic volumes would restrain transit priority implementation and allows identifying the required lane and signal timing modifications to the signalized intersection that for transit signal priority implementation would assure effective traffic operations under future conditions. Impact of Auxiliary Lane Extensions on Queue Jump Performance The adjacent through lane th percentile queue length is the maximum back of queue with th percentile traffic volumes. The th percentile queue length can be used to determine if the queue length in the adjacent through lane would extend to pass the auxiliary lane storage length, thus restricting buses from using of the right turn or separate lane to perform the queue jump at the signalized intersection. The difference in the auxiliary lane storage length and the th percentile queue length would give an estimate of the required lane lengthening for effective queue jump operations; the right turn traffic if a right turn lane is used is also analyzed to determine if the existing traffic volumes combine with the added bus traffic volumes would be appropriate to allow buses to clear the intersection with the proposed bus signal phase. TRB 0 Annual Meeting

10 0 0 In cases where queue jump implementation would give the highest travel time savings but the existing storage length of the auxiliary lane is not appropriate for effective BRT operations, a cost-benefit analysis is require to compare the benefits in reduction of operating cost vs. the increase in capital investment to lengthen the auxiliary lane. The existence of channelized right turn lanes restrain queue jump operations and require a cost-benefit analysis to determine the benefits of the require modifications to the existing intersection. The feasibility of the required modifications depends also on the availability of right-of-way and the presence of existing utilities. Example Application Background An urban area with one long, heavily traveled bus route is selected to study the feasibility of implementing transit signal priority strategies as a means to provide faster and more reliable transit service for a community exhibiting increasing congestion. The existing bus service in the corridor exhibits slow and unreliable operations, and for its high ridership, would exhibit significant passenger benefits and operational cost savings if a successful TSP strategy could be implemented. Intersection priority needs to be the focus for bus operational improvements given neither the lack of right-of-way for roadway widening to increase roadway capacity nor the ability of converting a general traffic lane to a bus lane due to capacity constraints. The travel savings associated with potential transit priority treatments for the signalized intersections along the corridor are then further evaluated to determine the feasibility of the proposed treatment and to select the appropriate locally preferred alternative for enhanced bus operations. For the purposes of this example, SYNCHRO.0 is used to evaluate the traffic operations at selected intersections using existing turning movement counts and signal timing plans. Two isolated intersections with different lane configurations and traffic volumes are selected and evaluated for TSP and queue jump feasibility. For optimal analysis, building a network of intersections for the corridor being analyzed is preferred to account for the impact of signal progression. 0 TRB 0 Annual Meeting

11 0 Sample Calculations Intersection A The existing lane configuration and traffic volumes for this location are show in Figure. 0 FIGURE Intersection A 0 FIGURE SYNCHRO.0 Analysis Results for Intersection A TRB 0 Annual Meeting

12 0 The bus service in the corridor being analyzed operates on the eastbound and westbound approaches at this signalized intersection. The overall existing level of service for Intersection A is C with a v/c ratio of 0. which implies that TSP is applicable and the presence of the exclusive right turn lane in the eastbound approach also implies that there is a queue jump opportunity for this approach and further analysis should be performed to determine the feasibility of the different options for transit priority treatments. The results shown in Figure are obtained from the analysis of the existing conditions performed using SYNCRHO.0 and are summarized in Table. The difference in the current delay for the through traffic lanes and the delay once the green time extension is implemented would give the travel time savings in seconds. The green time allocation for TSP is assumed to be recovered by other signal timing adjustments to avoid interrupting the signal coordination along the corridor. The difference in the delay for the general traffic lanes and the delay in the right turn lane for the eastbound approach once the estimated bus traffic volume is added, would give the travel time savings in seconds for queue jump operations. TABLE Intersection A - Analysis Results Summary Intersection A Approach Current Delay Through Traffic (sec) Right Turn Lane Storage Length (ft) TSP Travel Time Savings (sec) Queue Jump Travel Time Savings (sec) th Percentile Queue Length (ft) Westbound. N/A. N/A N/A Eastbound The results for the transit priority treatments at Intersection A imply that in the eastbound direction, the implementation of queue jump would result in the greatest travel time savings for bus operations; however, the th percentile queue length for the through traffic extends past the existing right turn storage lane of 00 ft. Further analysis is required to determine if the benefits in operating cost given by the travel time savings identified at this location would warrant the cost of lengthening the right turn lane to the appropriate length for effective queue jump operations. Other factors including availability of right of way and available construction funds should also be considered; for this particular case the cost of modifying the existing channelization in the right turn lane for the eastbound approach should also be part of the cost analysis. The travel time savings identified would also be used for modeling purposes and would be an important factor in determining the locally preferred alternative for BRT operations. The future conditions can be analyzed by adding a growth factor per approach to analyze traffic operations under projected future traffic volumes and following the same analysis steps, the feasibility for transit priority treatments can be identified. TRB 0 Annual Meeting

13 Intersection B The existing lane configuration and traffic volumes for this location are show in Figure. FIGURE Intersection B 0 FIGURE SYNCHRO.0 Analysis Results for Intersection B Intersection B is located in the same bus corridor as intersection A and would be operating on the eastbound and westbound approaches at this signalized intersection. The overall existing level of service for Intersection B is E with a v/c ration of.0 which implies that TSP is not applicable under the current lane configuration and traffic volumes. The presence of the exclusive right turn lane in the eastbound TRB 0 Annual Meeting

14 0 0 approach implies that there is a queue jump opportunity for this approach and further analysis should be performed to determine the feasibility of this transit priority treatment. The results shown in Figure are obtained from the analysis of the existing conditions performed using SYNCRHO.0 and are summarized in Table where travel time savings are identified. Intersection B Approach Current Delay Through Traffic (sec) Right Turn Lane Storage Length (ft) Queue Jump Travel Time Savings (sec) th Percentile Queue Length (ft) Westbound N/A N/A N/A N/A Eastbound TABLE Intersection B - Analysis Results Summary The results for the transit priority treatments at Intersection B imply that in the eastbound direction, the implementation of a queue jump would result in the only possibility for transit priority treatment to obtain travel time savings for bus operations; however, the th percentile queue length for the through traffic extends passing the existing right turn storage lane of 0 ft. The benefits in operating cost given by the travel time savings identified at this location would require further analysis to warrant the costs of lengthen the right turn lane to the appropriate length for effective queue jump operations. At the same time right of way and funds availability should also be considered. For this particular case, the analysis of modifications to the existing lane configuration to improve the level of service is performed resulting in the need of an extra eastbound through lane for a level of service D. This option can be presented to the agencies and stakeholders if the reconfiguration of the signalized intersection has been identified as a future improvement already approved by the appropriate agency. The signalized intersection improvement would open the possibility of implementing TSP at this location and add extra savings to bus operating costs. The results of the analysis if the proposed added eastbound through traffic lane was implemented are shown in Table Intersection B Approach Proposed Modification- Through Traffic Delay (sec) Right Turn Lane Storage Length (ft) TSP Travel Time Savings (sec) Queue Jump Travel Time Savings (sec) th Percentile Queue Length (ft) Westbound. N/A. N/A N/A Eastbound TABLE Intersection B Proposed Modification - Analysis Results Summary 0 Assuming that the eastbound through lane addition is feasible at this location, the results for the transit priority analysis at Intersection B imply that in the eastbound direction, the implementation of a queue jump would result in the greatest travel time savings for bus operations; however, the th percentile queue length for the through traffic extends past the existing right turn storage lane of 0 ft and further cost analysis is required. The future conditions should be analyzed by adding a growth factor per approach to determine traffic operations under projected future traffic volumes and following the same analysis steps, the feasibility for transit priority treatments can be identified. TRB 0 Annual Meeting

15 Identification of Appropriate Treatment from Operational Perspective The appropriate signal priority treatment is determined based on the analysis of the existing traffic operations at main intersections along the corridor being studied for enhanced bus operations. The levels of service and lane configuration are key factors in determining the applicability of TSP or queue jumps at each intersection. The travel time savings identified as a result of the implementation of transit priority treatments could be aggregated between segments or between bus stops in identifying relative priority for signal priority implementation. It is also important to analyze future conditions following the same steps to assure that bus operations would remain effective under future traffic changes. The cost analysis should be performed for different scenarios in which proposed modifications to the existing intersection lane configuration and/or right turn lane extensions are analyzed to determine if the savings in travel time would warrant the capital cost investment required. The travel time savings achieved by implementing signal priority treatments will result in an overall reduction in bus delays and capital cost savings by reducing the number of buses needed during the length of the day to provide service; operating costs are also reduced by increasing the efficiency in bus operations. Moreover, the delay in the general traffic lanes is also reduced improving the general traffic operations along the corridor. Reltationship in Interconnected Signal Systems It should be recognized that the methodology identified is a sketch planning tool for comparison of strategies at a specific intersection. It is recognized that in an interconnected signal system, adding travel time savings across intersections calculated independent of one another may underestimate or overestimate overall corridor savings when TSP is applied at multiple intersections, pending the impact on signal progression. If there is an opportunity to apply simulation modeling across a number of intersections, a more detailed assessment of travel time savings could be conducted, including more specific consideration of general traffic impacts. The results obtained from the analysis when applying the methodology presented in this paper, could be used as input to a more detailed simulation model. Conclusion Priority strategies for bus transit have the purpose of expediting transit flow by reducing travel times and delay while providing a more reliable system that may attract new riders and increase transit capacity. The interference of general traffic when transit is operating in mixed traffic results in a reduction in speed and capacity. Intersection priority treatments reduce signal delay resulting in travel time savings to transit. The analysis presented in this paper is a sketch planning tool that allows determining the transit priority strategy for one or more signalized intersections along any specific corridor being evaluated for bus operations and determining the feasibility of the identified preferred strategy under current and projected traffic conditions, as well as the added benefits to warrant the added costs. It is a methodology that can be applied when more expensive simulation modeling is not possible. The methodology presented for evaluating the impacts of TSP treatments on traffic operations by comparing various treatment options provides a decision framework that can be applied when analyzing the effectiveness of transit priority treatments in travel time savings for bus operations. TRB 0 Annual Meeting

16 REFERENCES Kittelson & Associates, Inc., Herbert S. Levinson Transportation Consultants. TCRP Report Bus Rapid Transit Practitioner s Guide. In Transit Cooperative Research Program, 00 Parsons Brinckerhoff, TCRP Synthesis Report, Bus and Rail Preferential Treatments in Mixed Traffic, In Transit Cooperative Research Program 00. TRB 0 Annual Meeting